The use of organic peroxides to crosslink (i.e., vulcanize) a wide variety of polymer resins is well known in the art.
It is also known to stabilize polymer compositions, including crosslinked polymer compositions against oxidative degradation through the incorporation of low concentrations of antioxidant compounds in the compositions. Oxidative degradation is a process involving the initiation, propagation and termination of free radicals leading to polymeric oxidation. For example, in Modern Plastics Encyclopedia, 88, pp. 127-128, 1988, it is disclosed that during the propagation phase, radicals react with oxygen to form peroxy and alkoxy radicals which abstract hydrogen from the polymer chain to form unstable hydroperoxides, alcohols, and new hydrocarbon free radicals. These free radicals can once again combine with oxygen to continue this cycle. Such degradation normally continues until a termination reaction occurs. Typically, as the degradation cycle occurs, a loss in tensile and elongation properties of the crosslinked system with time is observed. This loss is heightened at elevated temperatures.
Stabilization of polymeric compositions from the deleterious effects of oxidation may be achieved by the addition thereto of antioxidants which provide increased opportunities for termination reactions and/or prevent the formation of free radicals. Conventionally, a mixture of primary and secondary antioxidants are used in order to stabilize polymer compositions. The combination of primary and secondary antioxidants may be synergistic, i.e., the combination of the two is more effective than the individual components.
Primary antioxidants are those that scavenge free radicals and inhibit oxidation via the action of chain propagating radicals. Sterically hindered phenols, hindered amine light stabilizers, quinolines and secondary amines are commonly used primary antioxidants.
Secondary antioxidants function by decomposing peroxides and/or hydroperoxides into nonradical products. Commonly used secondary antioxidants include compounds containing phosphorus and sulfur.
Certain organic sulfides have been used as antioxidants and stabilizers in polymer compositions. For example, U.S. Pat. Nos. 3,652,680 and 3,772,246 teach the use of cycloalkane sulfides as antioxidants in non-crosslinked polyolefin resin compositions. Likewise, European Patent Application Publication No. 177784, published Apr. 16, 1986, discloses the use of cycloalkane bis(alyklsulfides) as ultraviolet light stabilizers in polyolefin resins. However, the use of such organic sulfide antioxidants in crosslinked polyolefins has heretofore not been known in the art.
Organic peroxides are generally described as oxidizing agents and antioxidants are known as peroxide decomposers. Therefore, it would be expected that combinations or blends of peroxides and antioxidants would be highly unstable. It is also well known that antioxidants can act as radical "traps". Thus, during crosslinking reactions, one would expect the crosslinking efficiency of the peroxide to be significantly reduced by the presence of an antioxidant. For example, S. C. Martens' article on chemically crosslinked polyethylene (The Vanderbilt Rubber Handbook, pp. 308-318, 1978), describes the effects of antioxidants on the crosslinking efficiency of organic peroxides. This reference teaches that antioxidants, particularly the phenolic-type antioxidants, have a significant detrimental effect on the crosslinking efficiency of peroxides. Similarly, results reported in The Vanderbilt Rubber Handbook, pp. 528-532, 1978, teach that the inclusion of antioxidants in peroxide crosslinked polymers and elastomers (e.g., EPM, EPDM) leads to reduced initial tensile properties. Thus, it would be expected that the inclusion of commercial antioxidants in polymeric and elastomeric resins crosslinked with a peroxide would detrimentally effect the peroxide.
Accordingly, it can be seen that there is a need for compositions which will effectively stabilize and crosslink polyolefins, allowing one to obtain good initial properties and property retention upon aging without adversely effecting the crosslinking efficiency of the peroxide.